1. Field of the Invention
This invention relates to an active matrix display device, especially to an active matrix display device having a plurality of retaining circuits provided for each of the pixel elements.
2. Description of the Related Arts
There has been a great demand in the market for portable communication and computing devices such as a portable TV and cellular phone. All these devices need a small, light-weight and low-consumption display device, and development efforts have been made accordingly.
FIG. 6 shows a circuit diagram corresponding to a single pixel element of a conventional liquid crystal display device. A gate signal line 51 and a drain signal line 61 are placed on an insulating substrate (not shown in the figure) perpendicular to each other. A pixel element selection TFT 65 connected to the two signal lines 51, 61 is formed near the crossing of the two signal lines 51, 61. The source 65 of the pixel element selection TFT 65 is connected to a pixel element electrode 17 of the liquid crystal 21.
A storage capacitor element 85 holds the voltage of the pixel element electrode 17 during one field period. A terminal 86, which is one of the terminals of the storage capacitor element 85, is connected to the source 65 of the pixel element selection TFT 65, and the other terminal 87 is provided with a voltage common among all the pixel elements.
When a gate signal is applied to the gate signal line 51, the pixel element selection TFT 65 turns to an on-state. Accordingly, an analog image signal from the drain signal line 61 is applied to the pixel element electrode 17, and the liquid crystal 21 through the pixel element electrode 17, and the storage capacitor element 85 holds the voltage. The voltage of the image signal is applied to the liquid crystal 21 through the pixel element electrode 17, and the liquid crystal 21 aligns in response to the applied voltage for providing a liquid crystal display image. Disposing the pixel elements as a matrix as described above provides a basic configuration of a LCD.
The conventional LCD is capable of showing both moving images and still images. There is a need for the display to show both a moving image and a still image within a single display. One such example is to show a still image of a battery within area in a moving image of a cellular phone display to show the remaining amount of the battery power.
However, the configuration shown in FIG. 6 requires a continuous rewriting of each pixel element with the same image signal at each scanning in order to provide a still image. This is basically to show a still-like image in a moving image mode, and the scanning signal needs to activate the pixel element selection TFT 70 by the gate signal at each scanning.
Accordingly, it is necessary to operate a driver circuit which generates a drive signal for the gate signals and the image signals, and an external LSI which generates various signals for controlling the timing of the drive circuit, resulting in a consumption of a significant amount of electric power. This is a considerable drawback when such a configuration is used in a cellular phone device, which has only a limited power source. That is, the time a user can use the telephone under one battery charge is considerably short.
Japanese Laid-Open Patent Publication No. Hei 8-194205 discloses another configuration for display device suited for portable applications. This display device has a static memory for each of the pixel elements. FIG. 7 is a plan view showing the circuit diagram of the active matrix display device with a retaining circuit disclosed in Japanese Laid-Open Patent Publication No. Hei 8-194205. A plurality of gate signal lines 51 and reference lines 52 are disposed in a predetermined direction. And a plurality of drain lines 61 are disposed in the direction perpendicular to the predetermined direction. Between a retaining circuit 54 and a pixel element electrode 17, a TFT 53 is formed. By displaying image based on the data retained in the retaining circuit, the operation of a gate driver 50 and a drain driver 60 is stopped for the reduction of the electric power consumption.
FIG. 8 shows a circuit diagram corresponding to a single pixel element of the liquid crystal display device. On a substrate, the pixel element electrode is disposed in a matrix configuration. Between the pixel element electrodes 17, the gate signal line 51 and the drain signal line 61 are placed perpendicular to each other. The reference line 52 is disposed parallel to the gate signal line 51, and the retaining circuit 54 is formed near the crossing of the gate signal line 51 and the drain signal line 61. A switching element 53 is formed between the retaining circuit 54 and the pixel element electrode 17. A static memory (Static Random Access Memory: SRAM), in which two inverters 55 and 56 are positively fed back to each other, works as the retaining circuit for holding the digital image signal. Since the SRAM dose not need to refresh the memory for retaining the data, the SRAM, which is different from DRAM, is suitable for the display device.
In this configuration, the switching element 53 controls the resistance between a reference line and a pixel element electrode 17 in response to the divalent digital image signal held by the static memory and outputted from the retaining circuit in order to adjust the biasing of the liquid crystal 21. The common electrode, on the other hand, receives an AC signal Vcom. Ideally, this configuration does not need refreshing the memory when the image stays still for a period of time.
However, when the static RAM is used in the retaining circuit 54, the number of the required transistors of the retaining circuit is 4 or 6, resulting in the enlargement of the circuit. Also, since the static RAM occupies the space between the pixel element electrodes 17, the area for the pixel element electrode reduces accordingly. As a result; the light manipulation area of the liquid crystal display device is limited, and the size of a single pixel element is larger than it is desired for the device design.
This invention is directed to the size reduction of the picture element or the increase of the light manipulation area of a display device with a retaining circuit for holding the data in response to the pixel element voltage.
This invention provides an active matrix display device having a plurality of pixel element electrodes disposed as a matrix, a plurality of retaining circuits disposed for the pixel element electrodes for holding a voltage determining an application of a voltage to the pixel element electrode, and a power line which supplies a predetermined voltage to the retaining circuit. The power line extends in one direction of the matrix and is commonly used by the retaining circuits of the pixel element electrodes forming a line of the matrix in the same direction. This power line is also used by the retaining circuits of the pixel element electrodes forming another line of the matrix in the same direction. The two lines are next to each other.
In this configuration, the number of the power lines of the active matrix display device can be reduced in half comparing to the case where the power line is provided for each of the rows. Thus, it is possible to achieve the size reduction of the pixel element, leading to the overall size reduction of an active matrix display device.
The active matrix display device of this invention also provides an active matrix display device including a plurality of pixel element electrodes disposed as a matrix, a plurality of gate signal lines disposed in the row direction of the matrix, a plurality of drain signal lines disposed in the columnar direction of the matrix. This active matrix display device also includes a first display circuit which supplies a signal corresponding to the image signal fed from the drain signal line to the pixel element electrode selected by the scanning signal inputted from the gate signal line; and a second display circuit having a retaining circuit which retains a voltage of the image signal fed from the drain signal line in response to the scanning signal inputted from the gate signal line. This second display circuit supplies a signal corresponding to the voltage held by the retaining circuit to the display electrode. The active matrix device of this invention further includes a circuit selection circuit which selectively connects one of the first and second display circuit to the drain signal line in response to a circuit selection signal, and a power line which supplies a predetermined voltage to the retaining circuit. This power line extends in one direction of the matrix and is commonly used by the retaining circuits of the pixel element electrodes forming a line of the matrix in that direction. The power line is also used by the retaining circuits of the pixel element electrodes forming another line of the matrix in the direction. The two lines are next to each other in the matrix.
In this configuration of the active matrix display device, which can select either the first or second display circuit, the number of the power lines can be reduced in half comparing to the case where the power line is disposed for each of the rows. Thus, it is possible to achieve the size reduction of the pixel element, leading to the overall size reduction of the active matrix display device.
In a preferred embodiment, each of the retaining circuits is connected to at least two driver power lines, which extend in one direction of the matrix and which provide different kind of voltage to each of the retaining circuit. Also, at least one of the driver power lines is commonly used for a plurality of the pixel elements adjacent in the other direction of the matrix.
Additionally, each of the retaining circuits is connected to at least two reference power lines, which extend in one direction of the matrix and each of which provides different kind of reference voltage to the retaining circuit. And the retaining circuit selects the reference voltage based on the retained data and supplies it to the pixel element electrode. Also, at least one of the reference power lines is commonly used for a plurality of the pixel elements adjacent in the other direction of the matrix.
Furthermore, the commonly used power line supplies the same voltage to all the retaining circuit.
Also, the commonly used power line is disposed near the pixel elements adjacent to each other in the other direction of the matrix. In the adjacent pixel elements in the other direction of the matrix, the retaining circuits are disposed symmetrically on the opposite side of the power line with the axis being between the adjacent pixel elements in the other direction of the matrix.